This application is based on applications Nos. 2001-174547 and 2001-174548 filed in Japan, the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a color conversion table generation method, a color conversion method, and a color conversion program. Particularly, the present invention relates to a color conversion table generation method, a color conversion method, and a color conversion program to convert color data represented by a certain color space into color data represented by another color space.
2. Description of Related Art
In order to achieve correct color reproduction on a printer, it is necessary to obtain the correct values of CMYK (Cyan, Magenta, Yellow and Black) that can represent an applied color (for example, L*a*b* that does not depend on the apparatus).
The most conventional CMYK conversion generally effected is CMY conversion+GCR (Gray Component Replacement) (or UCR (Under Color Removal)). This includes the processes of, as shown in FIG. 28, converting color data (L*a*b*) into the three colors of CMY, generating K data from the CMY data, and correcting the CMY data from the produced K data.
This conversion method is widely employed due to the fact that conversion to CMY can be conducted easily, and that a moderate image quality can be readily achieved by just adding black to the region of low lightness.
However, various problems are noted as set forth below when color conversion is conducted according to the conventional method.
The color reproduction range by CMY is smaller than the color reproduction range of CMYK (for example, by the combination of the three colors of CMY, umber brown is achieved instead of black). Therefore, in the conversion process from L*a*b* to CMY, there are some colors that cannot be represented by CMY even if reproduction in CMYK is allowed.
As to correcting the values of CMY based on the produced K data, it is difficult to identify the K component amount in CMY, and determine how much the CMY is to be corrected when K data is produced since the actual ink property is complex. In practice, conversion is carried out empirically (or by trial and error).
Also, in the case where an image on a display is to be printed out through a printer in a computer system, the RGB (Red, Green and Blue) signal output with respect to the display must be converted into the CMYK (Cyan, Magenta, Yellow and Black) signal employed for the printer output. Since both the RGB signal of the display and the CMYK signal of the printer depend on the property of the apparatus, it is necessary to first represent colors with the colorimetric system that does not depend on the property of the apparatus for the purpose of achieving accurate color conversion.
Specifically, the RGB signal is converted into the colorimetric system such as L*a*b* that does not depend upon the property of the apparatus, and then further converted into the CMYK signal.
In general, the color range that can be reproduced differs between a display and a printer. Therefore, an operation of substituting a color that cannot be represented through the printer with a color that can be represented (gamut compression) is necessary.
The conventional technique of gamut conversion is disclosed in, for example, Japanese Patent Laying-Open No. 10-84487. This method effects conversion so that the color difference between color data of the image prior to conversion and the color data of the image after conversion is reduced.
FIG. 29 is a diagram to describe a specific method of gamut compression. The L*a*b* space is viewed from the direction of the L* axis. The hatched region in FIG. 29 represents the gamut.
In the case where image data that is to be represented is located at the initial point of the arrow in FIG. 29, that data must be converted into data residing in the gamut since the data is located outside the gamut. Here, a color that cannot be reproduced is converted into a reproducible color by shifting the image data from the initial point of the arrow to the end point of the arrow.
In order to minimize the color difference, the end point of the arrow is set so that the distance from the initial point to the end point of the arrow is as short as possible.
In the above-described gamut compression method, problems called xe2x80x9cblue-shiftxe2x80x9d and xe2x80x9clightning rod effectxe2x80x9d are noted. Blue-shift is a phenomenon in which the color of bright blue outside the gamut is converted into the color of purple that has small color difference instead of the desirable conversion biased towards cyan. The lightning rod effect is a phenomenon in which data after conversion is congregated at the acute area of the gamut.
Various methods have been proposed to avoid such problems (for example, refer to Japanese Patent Laying-Open Nos. 10-258548 and 10-290375).
However, these methods require the accurate determination of the boundary of the gamut. The process inside the gamut and the process outside the gamut are independent of each other, so that the amount of calculation is increased. Furthermore, although appropriate conditions of constraint must be set so that there is no discontinuity in the data inside the gamut and outside the gamut, this setting is extremely difficult since the boundary plane to which the constraint condition is to be assigned is a three dimensional free-form surface.
An object of the present invention is to provide a color conversion table generation method, a color conversion method, a color conversion program, and an image processing apparatus that can effect better color reproduction without having to depend on empirical rules or trial and error.
Another object of the present invention is to provide a color conversion table generation apparatus and color conversion apparatus that can effect conversion readily and of enhanced appearance.
According to an aspect of the present invention, a color conversion table generation method includes the steps of: (a) printing out a plurality of colors determined in a color space formed of a plane indicating a cyan component and a magenta component, a plane indicating a magenta component and a yellow component, a plane indicating a yellow component and a magenta component, and a gray component that is not in parallel with any of these planes; (b) reading out color of each color printed out at step (a); and generating a table in which each colorimetric result obtained at step (b) and each color determined on the color space are in correspondence.
According to another aspect of the present invention, a color conversion table generation apparatus includes a printer for printing out a plurality of colors determined in a color space formed of a plane indicating a cyan component and a magenta component, a plane indicating a magenta component and a yellow component, a plane indicating a yellow component and a cyan component, and a gray component that is not in parallel with any of these planes; a colorimetry portion for reading out color of each color printed out at the printer; and a table generation portion for generating a table in which each colorimetric result obtained at the colorimetry portion and each color printed out by the printer and determined on the color space are in correspondence.
According to a further aspect of the present invention, a color conversion method conducts color conversion using a table in which each point of a first color space and each point of a second color space are set in correspondence. The first color space can represent a color not depending on the apparatus. The second color space is formed of a plane indicating the cyan component and the magenta component, a plane indicating the magenta component and the yellow component, a plane indicating the yellow component and the magenta component, and a gray component that is not in parallel with any of these planes.
According to still another aspect of the present invention, a color conversion apparatus includes a color conversion portion for color conversion using a table in which each point in a first color space and each point in a second color space are set in correspondence. The first color space can represent a color not depending on the apparatus. The second color space is formed of a plane indicating the cyan component and the magenta component, a plane indicating the magenta component and the yellow component, a plane indicating the yellow component and the magenta component, and a gray component that is not in parallel with any of these planes.
According to a still further aspect of the present invention, an image processing apparatus forming an image that is to be printed out with a black colorant and a plurality of color colorants includes an image processing portion for increasing the usage amount of at least one color colorant among the plurality of color colorants with the usage amount of the black colorant the same as the color that is to be represented becomes blacker in order to represent a blacker color from a state where the usage amount of the black colorant is at its maximum.
According to yet a further aspect of the present invention, an image processing method forms an image to be printed out with a black colorant and a plurality of color colorants. In order to represent a blacker color from the state where the usage amount of the black colorant is at its maximum, the usage amount of at least one type of color colorant among the plurality of color colorants is gradually increased with the usage amount of the black colorant the same as the color to be represented becomes blacker.
According to yet another aspect of the present invention, a color conversion table is used in an image processing apparatus to form an image to be printed out with a black colorant and a plurality of color colorants. The color conversion table stores information to gradually increase the usage amount of at least one type of color colorant among the plurality of color colorants with the usage amount of the black colorant the same as the color to be represented becomes blacker, in order to represent a blacker color from the state where the usage amount of the black colorant is at its maximum.
According to yet a still further aspect of the present invention, a color conversion table generation apparatus includes an output portion for providing a plurality of types of colors using color data in a first space; an acquirement portion for acquiring color data in a second color space of each output color by reading out color of each color output from the output portion; a conversion portion for conducting conversion to deform the second color space on the acquired color data; and a generation portion for generating a table in which the output color data of the first color space and the color data converted by the conversion portion are in one-to-one correspondence.
According to an additional aspect of the present invention, a color conversion table generation method includes the steps of: (a) providing a plurality of types of colors using color data in a first color space; (b) acquiring color data in a second color space of each output color by reading out color of each color output at step (a); (c) carrying out a conversion to deform the second color space on the color data acquired at step (b); and (d) generating a table in which color data of the first color space output at step (a) and color data converted at step (c) are in one-to-correspondence.
According to still an additional aspect of the present invention, a color conversion apparatus includes a storage portion for storing a table indicating corresponding relationship between data of a first color space representing color depending on the apparatus and data of a second color space that is a deformation of a color space that represents a color that does not depend on the apparatus; an input portion for applying color data in the color space that does not depend upon the apparatus; a conversion portion for converting the color data applied by the input portion into color data of the second color space; and a reader portion for reading out from said storage portion data of the first color space corresponding to color data converted by the conversion portion.
According to yet a further additional aspect of the present invention, a color conversion method for color conversion using a storage portion in which is stored a table that indicates the corresponding relationship between data of a first space depending upon the apparatus and data of a second space which is a deformation of a color space that does not depend upon the apparatus includes the steps of: (a) applying color data of a color space that does not depend upon the apparatus; (b) converting the color data input at step (a) into color data of the second color space; and (c) reading out from the storage portion data of the first color space corresponding to the converted color data.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.